MinXSS Science Nugget 2

Solar abundance comparison between eruptive and confined flares

2017 June 2

James Paul Mason and the MinXSS Science Team

Inside the solar soft x-ray spectrum that MinXSS measures, there are several emission lines that are the signatures of certain ions, Fe XXV for example. This emission of photons only happens if the responsible ion is actually present in the solar plasma. At progressively higher temperatures, more and more electrons get kicked out of an atom, resulting in a new species of ion. So, by measuring the shape and size of an emission line, we can get a sense of how abundant that ion is in the solar atmosphere and of what temperature the plasma is. By combining our measurements with CHIANTI (an aggregated atomic database with modeling tools), we can quantify temperatures and abundances.

First we start with a MinXSS spectrum. We then fit CHIANTI synthetic spectra to the measurement, varying the abundance and temperature in the model. We fit two different temperatures, a hot component (> 10 MK) and if there are any residuals at low energies, we add in a cooler component as well (~few MK). We can then derive abundance parameters from the fit. For this study, our main interest is in the difference between coronal and photospheric abundances during flares. The ratio of low-first-ionization potential (FIP) abundances is taken from the literature as 2.138. Separately, we watched a bunch of SDO/AIA movies of flares to determine whether they were eruptive (plasma gets ejected) or confined (see local brightening but no significant plasma flow) by eye. Eruptive events are often caused by charged particles getting accelerated in the corona during a magnetic reconnection, which remain confined to the magnetic fields and so stream down the field lines until they impact the denser chromosphere. This causes an explosion in the chromosphere and plasma can be ejected.

Our expectation is that for eruptive events, we should see the abundance level drop from coronal to photospheric as the plasma from lower down in the atmosphere gets kicked up into the corona and heated. The figures here show an example of each that tentatively confirm this hypothesis. We’ve only analyzed a handful of cases so far and more analysis is needed. In particular, we need to look at more events, and the abundance fits need to use more than just the Fe XXV line for fitting in order to be sensitive to lower temperature plasma. This initial examination of the MinXSS spectra for a few confined and eruptive flares suggests that the abundance change during flares could be different depending on which type of flare, and if so, then different coronal heating contributions might be involved in the different flare types.